1. Field of the Invention
The invention relates to a radome with an integrated plasma shutter that includes a plasma-guiding layer and electrodes for plasma excitation.
2. Discussion of Background Information
Antennas (e.g., of radar sets or of other sensors or communication devices) on aircraft, but also on ships or ground stations are often sealed off from the environment by electromagnetically transparent covers, so-called radomes. Problems exist with radomes of military aircraft in that the electromagnetic transparency of the radome necessary for the operation of the antenna system lying beneath it makes it more or less permeable for other undesirable electromagnetic waves. Consequently the following results:
The radar signature of a radome with antenna lying beneath it is generally much higher due to the reflections from the radome interior than the radar signature that would result from the exterior geometry of the radome with conductive or radar-absorbing embodiment.
The antenna and the surrounding installations are acted on unimpeded by interfering radiation penetrating into the radome. This interfering radiation can either be directed to the antenna and the surrounding installations in a targeted manner (e.g., by an interfering transmitter) or originate from any sources (e.g., from other radar equipment or other radiation sources).
This problem has been alleviated or prevented completely by a radome embodied or formed to be electromagnetically transparent only in the desired frequency range and/or only at the times at which the antenna is active.
In order to achieve this, various methods are already known:
So-called frequency-selective radomes exhibit a dependency of the electromagnetic transparency as a function of the frequency, so that its own working frequency range is allowed through the radome in a more or less unimpeded manner, but other frequency ranges are blocked or substantially damped. Depending on the design and requirement, the frequency filter formed by the frequency-selective radome can be a band-pass filter, a high-pass filter, or a low-pass filter.
Switchable radomes can be switched backwards and forwards between an electromagnetically transparent state and an electromagnetically reflecting or absorbing state.
Frequency-selective radomes can be realized with different methods, depending on the requirement profile. In particular, the use of one or more thin structured metal layers, so-called frequency-selective layers (FSL), which have a pronounced frequency dependence of the electromagnetic transparency, is known, e.g., from U.S. Pat. No. 6,218,978.
Switchable radomes can be realized in different ways. Mechanical shutter systems are thus known, in which shades are slid in front of the antenna. Another approach lies in inserting layers with variable surface impedance into the radome, such as through the use of pin diodes or of photoresistances according to German Application No. DE 39 20 110 C2. The variable layer can thereby act in an electrically conductive and thus reflecting or electrically insulating and thus transparent manner depending on the switching status.
Another approach for realizing a variable layer is the use of a layer or a volume of plasma. A plasma layer is electrically conductive, and a sufficiently high electrical conductivity for the reflection or damping of electromagnetic waves can be achieved depending on the charge density in the plasma. This behavior is already used for plasma-based antennas; see, e.g., U.S. Pat. No. 5,182,496. The desired switching action can be achieved by switching the plasma on and off.
With a plasma shutter, there is in principle the question of the integration of the plasma volume into the radome structure. A plasma shutter system has become known from the Russian Academy of Sciences, in which the area between the antenna and radome is filled with a plasma. Another concept according to German Application No. DE 43 36 841 C1 is based on plasma-filled tubes in front of the antenna. The plasma in the tubes is generated by lateral electrodes not lying in the visual range of the antenna. The disadvantage of the latter concept is the fact that the shutter element represents a separate component with respect to the radome, so that the stability of the radome is reduced by the installation of the shutter element. The integration of the shutter element into the radome furthermore leads to additional radar scattering centers on the radome, which has an unfavorable effect on the radar signature. Furthermore, the two electrodes for plasma generation are arranged laterally on the narrow sides of the plasma-guiding layer, which reduces the homogeneity of the electromagnetic field within the plasma-guiding layer.